1. Field of the Invention
The present invention relates to a brushless motor that is inexpensive and has greatly improved output torque.
2. Description of the Related Art
Attempts have been made to improve the performance of a brushless motor by improving the performance of a magnet, using a low-loss iron core material, or improving a coil fill factor, and thus reducing copper loss. For example, a direct drive motor for washing machines is directly connected to a load with an acceleration/deceleration mechanism, such as a gear, not being provided at an output shaft of the motor. Consequently, it is necessary for the motor to output large torque at a low speed considering the characteristics of the load. To this end, it is necessary to restrain the increase of temperature at a low-speed operation. In the related art, the motor is multipolarized (for example, 48 poles for a rotor and 36 slots for a stator) to shorten the circumferential length of coils of the stator and reduce a resistance value. That is, when one equivalent coil is equally designed, one equivalent resistance value is reduced by increasing poles (slots) of the stator, such that the thickness of the teeth is reduced as compared with non-multipolarization, and the circumferential length per turn of a coil is shortened as much as the thickness of the teeth.
Also, a demand to increase the torque and decrease the costs of the brushless motor has risen, and the correspondence based on the above-described construction has reached the limit. For these reasons, therefore, there has been developed a brushless motor of which the basic structure is greatly modified in recent years. For example, a vernier type motor mainly used for a stepping motor (see Japanese Patent No. 3140814) is considered to be applied to a brushless motor disclosed in Japanese Patent Application Publication No. 2006-61326.
As shown in FIG. 12, a brushless motor A100 includes a rotor A1 having magnetized surfaces A11 alternately magnetized as N and S poles in the rotational direction and a stator having a plurality of salient poles A3 on each of which a coil A31 is wound. The salient poles A3, which are driven at a different phase, are offset from the N and S poles. Tip end surfaces of the salient poles A3 are opposite to the magnetized surfaces A11 in the radial direction. At each tip end surface are alternately formed a groove A33 and protrusions A32 extending in the axial direction and having a width in the rotational direction approximately equal to that of the N pole or the S pole. The coil A31 of the corresponding salient pole A3 is electrically conducted, depending upon the position of the rotor A1, to rotate the rotor A1.
In this structure, a gap A6 between each groove A33 and the corresponding magnetized surface A11 is greater than a gap A6 between each protrusion A32 and the corresponding magnetized surface A11, with the result that magnetic flux linked to each tooth is improved, and high torque is achieved as compared to the conventional brushless motor A100 in which the salient poles A3 are opposite to the respective poles although the brushless motor has the same coil (the number of windings of one equivalent coil) (See FIG. 12).
However, the brushless motor A100 disclosed in Japanese Patent Application Publication No. 2006-61326 has difficulty achieving high torque. As shown in FIG. 12, for example, the gap distance between each S pole opposite to the center of a U-shaped coil and the corresponding salient pole A3 in the radial direction is greater than that between the N poles at opposite sides of the S pole. As a result, magnetic flux of the S pole is generated, although the magnetic flux of the S pole is not greater than that of the N poles, and the magnetic flux of the S pole becomes leakage magnetic flux M flowing to the N poles at the opposite sides of the S pole. This leakage magnetic flux M greatly reduces effective magnetic flux linked to the U-shaped coil.